Comfort and road handling performance of a passenger car are mainly determined by the damping characteristic of the shock absorbers. Passive shock absorbers have a fixed damping characteristic determined by their design. Depending on the road excitation, however, it is desirable to adjust this characteristic to increase performance. Semi-active and active suspension systems offer the possibility to vary the damper characteristics along with the road profile (e.g., by changing the restriction of one or two current controlled valves). An active shock absorber has the additional advantages that negative damping can be provided and that a larger range of forces can be generated at low velocities, thereby potentially allowing an increase in system performance.
Several theoretical linear and nonlinear techniques have been described to control a car using an active suspension. These techniques apply linear control strategies based on linear physical car models consisting of lumped masses, linear springs and dampers, and an active shock absorber modelled as an ideal force source. However, real car dynamics are much more complex and active shock absorbers are not ideal force sources but have a complex nonlinear dynamic behaviour. As a result of these unrealistic assumptions, these prior art linear control approaches are not appropriate for practical applications.
Nonlinear control strategies such as linear parameter varying gain scheduling and backstepping have been applied to active suspension systems and validated by means of simulations only. These controllers are based on a linear or nonlinear physical car model in combination with a nonlinear physical damper model. These models have a large number of parameters. The experimental identification of these model parameters is a complex (non-convex optimization) problem. In addition, design and tuning of the above mentioned nonlinear controllers are not straightforward. Basically, the use of nonlinear models and controllers leads to very time-consuming designs, since no standard techniques or software tools are available. Finally, the implementation of these controllers is too complex for practical use in a passenger car control system.
Based on the foregoing, there is a need for an improved active suspension system that utilizes linear control.